U.S. patent number 5,268,920 [Application Number 07/788,674] was granted by the patent office on 1993-12-07 for system for end-pumping a solid state laser using a large aperture laser diode bar.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the Navy. Invention is credited to Leon Esterowitz, James G. Lynn, Robert C. Stoneman.
United States Patent |
5,268,920 |
Esterowitz , et al. |
December 7, 1993 |
System for end-pumping a solid state laser using a large aperture
laser diode bar
Abstract
An optical system for end-pumping the gain medium of a
three-level or a self-terminating solid state laser with the
optical output from a wide aperture laser diode bar is disclosed.
In a preferred embodiment, the optical system includes: a laser
diode bar for emitting from an elongated emissive area thereof a
bright light having a lateral divergence and a transverse
divergence; and an optical assembly disposed between the laser
diode bar and the three-level solid state laser for collecting and
focusing the bright light into a relatively small high-intensity
spot to end-pump the gain medium of the three-level or
self-terminating solid state laser.
Inventors: |
Esterowitz; Leon (Springfield,
VA), Stoneman; Robert C. (Alexandria, VA), Lynn; James
G. (Alexandria, VA) |
Assignee: |
The United States of America as
represented by the Secretary of the Navy (Washington,
DC)
|
Family
ID: |
25145212 |
Appl.
No.: |
07/788,674 |
Filed: |
November 6, 1991 |
Current U.S.
Class: |
372/71;
372/101 |
Current CPC
Class: |
G02B
6/425 (20130101); H01S 3/09415 (20130101); H01S
5/4025 (20130101) |
Current International
Class: |
G02B
6/42 (20060101); H01S 3/0941 (20060101); H01S
5/40 (20060101); H01S 5/00 (20060101); H01S
003/091 (); H01S 003/092 () |
Field of
Search: |
;372/71,101 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Publication, Laser Diode Pumped 1-W CW Green Laser, by M. Oka et
al., Conence on Lasers and Electro-Optics, 1990 Technical Digest
Series, vol. 7, 21-25, May 1990, Anaheim, Ca., Paper CWC5,
1990..
|
Primary Examiner: Bovernick; Rodney B.
Assistant Examiner: Wise; Robert E.
Attorney, Agent or Firm: McDonnell; Thomas E. Jameson;
George
Claims
What is claimed and desired to be secured by Letters Patent of the
United States is:
1. An optical system for end-pumping a gain medium containing
active dopant ions in a resonant optical cavity of a solid state
laser, said optical system comprising:
laser pumping means for producing from an elongated area thereof at
least a 10 watt output bright light with given lateral and
transverse divergences; and
optical means in proximity to said laser pumping means for focusing
said output bright light into a relatively small high intensity
spot at one end of said gain medium to end-pump said solid state
laser.
2. The optical system of claim 1 wherein:
said laser pumping means is pulsed to produce pulses of output
bright light.
3. An optical system for end-pumping a gain medium containing
active dopant ions in a resonant optical cavity of a solid state
laser, said optical system comprising:
laser diode pumping means for producing from an elongated area
thereof an output bright light with given lateral and transverse
divergences; and
optical means in proximity to said laser diode pumping means for
focusing said output bright light into a relatively small high
intensity spot at one end of said gain medium to end-pump said
solid state laser, and wherein:
said solid state laser is a three-level laser.
4. An optical system for end-pumping a gain medium containing
active dopant ions in a resonant optical cavity of a solid state
laser, said optical system comprising:
laser diode pumping means for producing from an elongated area
thereof an output bright light with given lateral and transverse
divergences; and
optical means in proximity to said laser diode pumping means for
focusing said output bright light into a relatively small high
intensity spot at one end of said gain medium to end-pump said
solid state laser, said optical means including:
a plurality of cylindrical lenses to substantially collimate said
output bright light; and
a plurality of spherical lenses to focus said substantially
collimated output bright light into said relatively small high
intensity spot.
5. The optical system of claim 4 wherein:
each of said plurality of cylindrical lenses and said plurality of
spherical lenses is coaxially aligned in sequence with the other
lenses.
6. The optical system of claim 4 wherein:
said laser diode pumping means includes a laser diode bar
containing a plurality of separate laser diode arrays.
7. An optical system for end-pumping a gain medium containing
active dopant ions in a resonant optical cavity of a solid state
laser, said optical system comprising:
laser diode pumping means for producing from an elongated area
thereof an output bright light with given lateral and transverse
divergences; and
optical means in proximity to said laser diode pumping means for
focusing said output bright light into a relatively small high
intensity spot at one end of said gain medium to end-pump said
solid state laser, and wherein:
said active dopant ions have physical properties such that the
end-pumping of said solid state laser produces a stimulated
emission corresponding to a three-level optical transition for said
active dopant ions.
8. The optical system of claim 7 wherein:
said active dopant ions are comprised of thulium.
9. The optical system of claim 7 wherein:
said active dopant ions are comprised of holmium.
10. A laser system comprising:
an active medium in a resonant optical cavity of a solid state
laser;
laser diode pumping means for emitting from an elongated emissive
area thereof at least a 10 watt bright light having a lateral
divergence and a transverse divergence; and
optical means disposed between said laser diode pumping means and
said solid state laser for focusing said bright light into a
relatively small high intensity spot to end-pump said solid state
active medium.
11. The laser system of claim 10 wherein:
said laser diode pumping means is pulsed to produce pulses of
bright light.
12. A laser system comprising:
an active medium in a resonant optical cavity of a solid state;
laser diode pumping means for emitting from an elongated emissive
area thereof a bright light having a lateral divergence and a
transverse divergence; and
optical means disposed between said laser diode pumping means and
said solid state laser for focusing said bright light into a
relatively small high intensity spot to end-pump said solid state
active medium, said optical means including:
a large numerical aperture, compound cylindrical lens 27 to
collimate the output bright light of said laser diode bar in said
transverse direction; and
a compound spherical lens to focus said collimated bright light
into said relatively small high intensity spot.
13. A laser system comprising:
an active medium in a resonant optical cavity of a solid state;
laser diode pumping means for emitting from an elongated emissive
area thereof a bright light having a lateral divergence and a
transverse divergence; and
optical means disposed between said laser diode pumping means and
said solid state laser for focusing said bright light into a
relatively small high intensity spot to end-pump said solid state
active medium, said optical means including:
three circular-cylinder plano-convex lenses;
one spherical biconvex lens; and
three spherical plano-convex lenses.
14. A laser system comprising:
an active medium in a resonant optical cavity of a solid state;
laser diode pumping means for emitting from an elongated emissive
area thereof a bright light having a lateral divergence and a
transverse divergence; and
optical means disposed between said laser diode pumping means and
said solid state laser for focusing said bright light into a
relatively small high intensity spot to end-pump said solid state
active medium, said optical means including:
a plurality of cylindrical lenses to substantially collimate said
bright light; and
a plurality of spherical lenses to focus said substantially
collimated bright light into said relatively small high intensity
spot.
15. The laser system of claim 14 wherein:
said laser diode pumping means includes a laser diode bar
containing a plurality of separate laser diode arrays.
16. A laser system comprising:
an active medium in a resonant optical cavity of a solid state;
laser diode pumping means for emitting from an elongated emissive
area thereof a bright light having a lateral divergence and a
transverse divergence; and
optical means disposed between said laser diode pumping means and
said solid state laser for focusing said bright light into a
relatively small high intensity spot to end-pump said solid state
active medium, said wherein:
said solid state laser is a three-level laser.
17. A laser system comprising:
an active medium in a resonant optical cavity of a solid state
laser, said active medium including a host material containing
active dopant ions, said active dopant ions having physical
properties such that the end-pumping of said solid state laser
produces a stimulated emission corresponding to a three-level
optical transistion for said active dopant ions;
laser diode pumping means for emitting from an elongated emissive
area thereof a bright light having a lateral divergence and a
transverse divergence; and
optical means disposed between said laser diode pumping means and
said solid state laser for focusing said bright light into a
relatively small high intensity spot to end-pump said solid state
active medium.
18. The laser system of claim 17 wherein:
said active dopant ions are comprised of thulium.
19. The laser system of claim 17 wherein:
said active dopant ions are comprised of holmium.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to lasers and particularly to a
system for focusing the output of a long aperture laser diode bar
into a small, high intensity spot to end pump a solid state
laser.
2. Description of the Prior Art
There is currently considerable interest in lasers capable of
generating near-infrared radiation for medical and remote sensing
applications. One laser operating in this region is the 2 .mu.m
Tm.sup.3+ 3 F.sub.4.sup..fwdarw.3 H.sub.6 system. The strong .sup.3
H.sub.6.sup..fwdarw.3 H.sub.4 absorption at 785 nm coincides with
the emissions of recently available, high-power laser diode bars.
Thus this laser can take advantage of the size, weight, reliability
and efficiency benefits offered by laser diode pumping. However,
the 2 .mu.m Tm.sup.3+ 3 F.sub.4.sup..fwdarw.3 H.sub.6 system is a
three-level laser, and is therefore subject to special
considerations.
In three-level lasers, every active ion within the laser mode
volume either contributes gain if it is in the excited state, or
loss if it is in the ground state. Hence, a three-level laser
normally has a higher threshold than a comparable four-level laser
system, where ground-state ions have negligible effect. While
considerable effort has been invested in developing methods for
using laser diode bars to pump four-level Nd.sup.3+ 3
F.sub.3/2.sup..fwdarw.4 I.sub.11/2 lasers, these configurations do
not lend themselves well to three-level lasers and other systems
exhibiting laser reabsorption losses.
Optimal side pumping requires a sufficiently high activator ion
concentration to absorb much of the incident pump radiation and an
active medium long enough to take advantage of the geometry of the
pump source. The resultant, nearly-uniform, pump deposition across
a laser rod requires a large laser mode volume to extract most of
the stored energy from the rod. The large number of activator ions
present within the mode volume results in such a high threshold for
three-level systems that side-pumping with laser-diode bars may be
practical only for very high-power lasers incorporating several
tens of diode bars. The threshold can be lowered to a more easily
attained level by reducing the activator ion concentration, which
reduces the mode-matching and coupling efficiency, or by reducing
the pump and laser mode volume, which is not easily reconciled with
side pumping.
The poor spatial mode-matching and poor coupling efficiencies
characteristic of workable laser-diode bar, side-pumped geometries,
when considered in parallel with the high probability of
re-absorption losses due to unpumped regions in the laser rod, and
the higher threshold of three-level lasers, renders side-pumping
with laser diodes impractical for most three-level lasers.
The excellent mode-matching afforded by end-pumping is particularly
advantageous to three-level lasers for ensuring that all active
ions within the laser mode volume are pumped, thus minimizing
re-absorption losses. High-efficiency operation of a diode-pumped
Tm.sup.3+ : YAG laser has been demonstrated in the prior art, but
the output power is limited by the pump power available from the
individual phased-array laser diodes needed to provide the high
quality beam necessary for good mode matching. What is needed at
the present time is a way to end-pump three-level lasers using
high-power laser-diode bars.
OBJECTS OF THE INVENTION
Accordingly, one object of the invention is to provide a system for
end-pumping a three-level solid state laser using a large aperture
laser diode bar.
Another object of the invention is to provide an optical system for
focusing the output of a large aperture laser diode bar into a
small high-intensity spot to end-pump a solid state laser.
A further object of the invention is to provide an optical system
for collecting and focusing the bright light from a laser diode bar
into a relatively small, high-intensity spot to end-pump a
three-level or self-terminating solid state laser.
SUMMARY OF THE INVENTION
These and other objects of the invention are achieved by providing
an optical system for end-pumping the gain medium of a three-level
or a self-terminating solid state laser with the optical output
from a wide aperture laser diode bar. The optical system includes:
a laser diode bar for emitting from an elongated emissive area
thereof a bright light having a lateral divergence and a transverse
divergence; and an optical assembly disposed between the laser
diode bar and the three-level solid state laser for collecting and
focusing the bright light into a relatively small high-intensity
spot to end-pump the gain medium of the three-level or
self-terminating solid state laser.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects, features and advantages of the invention,
as well as the invention itself, will become better understood by
reference to the following detailed description when considered in
connection with the accompanying drawings wherein like reference
numerals designate identical or corresponding parts throughout the
several views and wherein:
FIG. 1 is a simplified side view of a preferred embodiment of the
invention;
FIGS. 2A and 2B respectively represent top and side views of
exemplary optical lenses which may be utilized in the optical
assembly of FIG. 1;
FIG. 3 is a magnified view of focal point A shown in FIGS. 2A and
2B;
FIG. 4 illustrates the horizontal pump spot profile of the optical
pump power as a function of position along the lateral axis passing
through FOCAL POINT A; and
FIG. 5 illustrates an exemplary plot of the output energy of the
Tm.sup.3+ :YAG laser of FIG. 1 as a function of the absorbed input
energy derived from the optical energy focused at point A.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, FIG. 1 is a simplified side view of
the invention in which the output bright light of a large aperture
laser diode bar 11 is focused by a lens assembly 12 into a
relatively high intensity spot to end-pump a solid state laser
13.
The solid state laser 13 is comprised of a laser rod or active or
gain medium 14 disposed in a laser cavity 15 defined by
optically-aligned input and output reflective elements of mirrors
17 and 19. Reflective elements 17 and 19 oppose each other on a
common axis to form a reflective path therebetween.
For purposes of ease of understanding of this description, the
active medium or laser rod 14 will be selected to be a 12%
Tm.sup.3+ :YAG active medium, which enables the solid state laser
13 to produce a 2 .mu.m output when it is pumped with a 785 nm pump
beam from the laser diode bar 11. However, it should be understood
that any selected one of other active mediums could be used. In
such a case, a different laser diode bar 11 (as well as a different
lens assembly 12) may have to be used to match the output
wavelength of the laser diode bar 11 to the absorption band of the
active medium or laser rod 14.
The laser rod 14, which can have a thickness between 0.25 mm and 25
mm (millimeters), has an exemplary thickness in this description of
approximately 1 millimeters (mm) and also has flat and parallel
surfaces 21 and 23. The input surface 21 of the laser rod 14 has a
dichroic coating which operates as the input reflective element or
mirror 17. However, it should be understood that the mirror 17
could be a separate mirror spaced apart from the laser rod 14. The
dichroic coating or mirror 17 has a high transmission (over 76%) at
the pump wavelength of 785 nm and a high reflection (about 99%) at
a wavelength of about 2 microns. The second surface 23 of the laser
rod 14 has an anti-reflection coating 25 at about 2 microns.
The output reflective element or mirror 19, which defines the
output end of the laser cavity 15, is concave and has an exemplary
10 centimeter (cm) radius of curvature. This output mirror 19,
which is about 99.5% reflective at about 2 microns, is positioned
about 9 cm away from the reflector 17 and serves as an output
coupler to output a portion of the laser emission developed by the
laser rod 14 when it is pumped by the 785 nm pulsed laser emission
from the laser diode bar 11. The resulting cavity beam waist is 280
.mu.m in diameter.
A typical high power laser diode bar 11 which produces a pulsed, 50
watt, 785 nm output which may be used to end pump the solid state
laser 13 is a SDL-3240 laser diode bar manufactured by Spectra
Diode Laboratories of San Jose, Calif. With such a high power,
laser diode bar 11 used in FIG. 1, a heat sink 20 may be used to
cool the laser diode bar 11.
The laser diode bar 11 is typically very small--about 1 cm long, a
few 100 microns tall and about 1 mm wide. Along the face of the
laser diode bar 11, that is about 1 cm long and about a few 100
microns tall, there are about 20 laser diode arrays (not shown),
with each laser diode array containing about 30 laser diodes. All
of the laser diodes in all of the arrays are mounted in a line of
what amounts to a linear source of 600 laser diodes. All of these
diodes are simultaneously pulsed by their power supply (not shown)
to produce about 500 microsecond long pulses of light at a combined
output power of about 50 watts. The laser diode bar 11 emits no
light when it is not pulsed by its power supply.
Each individual laser diode in the laser diode bar 11 emits a
two-lobe pattern (not shown), With light coming out of the two
lobes. The two lobes of each of the 600 laser diodes are all in the
same plane as defined by the line of the 600 laser diodes and the
direction of emission. The centers between two lobes from a given
laser diode are longitudinally (or laterally) separated from each
other by about 10 degrees, with each of the two lobes having a
width or lateral divergence of about 4 degrees. In the orthogonal
(or transverse) direction, each of the 1200 lobes has a divergence
of about 45 degree full width at half maximum. All of the 1200
beams or lobes have about the same lateral and transverse
divergences. Thus, the laser diode bar Il appears to produce from
an elongated area thereof an output bright light with given lateral
and transverse divergences.
The lens assembly 12 utilizes a large numerical aperture, compound
cylindrical lens 27 to collimate the output of the laser diode bar
11 in the large-divergence (fast axis) direction, which is
perpendicular to the electric vector and perpendicular to the axis
of the laser rod 14. The lens assembly 12 then utilizes a compound
spherical lens 29 to focus the collimated beam to a small high
intensity spot, which is shown as FOCAL POINT A in each of FIGS. 2A
and 2B.
Referring now to FIGS. 2A and 2B, the lens assembly 12 will be
discussed in greater detail. The specific problem that the lens
assembly 12 has to solve is to collect all of the light from the
1200 lobes of the 600 individual diodes in the laser diode bar 11
and focus all of that light into a small high intensity spot. It
should be noted at this time that FIGS. 2A and 2B are drawings of
the ray trace through the optics of the lens assembly 12. Only the
two-lobe outputs of five of the 600 laser diodes in the laser diode
bar 11 are shown in FIGS. 2A and 2B for the sake of clarity.
However, it should be understood that substantially all of the
two-lobe outputs of the 600 laser diodes in the laser diode bar 11
would pass through the optics shown in FIGS. 2A and 2B.
FIG. 2A is a top view of the ray trace through the optics of the
lens assembly 12, showing the low-divergence (slow axis or 10
degree) direction. On the other hand, FIG. 2B is a side view of the
ray trace through the optics of the lens assembly, showing the
large or wide divergence (fast axis or 45 degree) direction, which
is perpendicular to the electric vector and perpendicular to the
axis of the laser rod 14. The 10 degree divergence shown in FIG. 2A
is very small when compared to the 45 degree divergence shown in
FIG. 2B. As a result, the elongated beam from the laser diode bar
11 is treated as already being substantially collimated in the
low-divergence (slow axis or 10 degree) direction. On the other
hand, no such assumption can be made for the 45 degree divergence
shown in FIG. 2B. Consequently, cylindrical optics are used to
collimate the elongated beam from the laser diode bar 11 in the
large divergence (fast axis or 45 degree) direction shown in FIG.
2B. Then, as stated above, a compound spherical lens is calculated
that will focus the resultant collimated beam to a small spot.
The configuration of the lens assembly 12 is then optimized, using
a commercially available ray tracing package to produce the
smallest spot from the elongated output of the laser diode bar 11.
The model used to represent the laser diode bar 11 during
optimization assumes that the laser diode bar 11 is a linear array
of lasers, each radiating having a full width at half maximum
(FWHM) of approximately 2 degrees in the slow direction and
approximately 45 degrees in the fast direction. Although
preliminary designs use arbitrary lenses, the final optimized
version uses only commercially available lenses. Included are
circular-cylinder plano-convex (PCX CYL) lenses 31, 32 and 33;
spherical biconvex (BCX SPH) lens 34; and spherical plano-convex
(PCX SPH) lenses 35, 36 and 37, as shown in FIGS. 2A and 2B. All of
the lenses 31-37 of the lens assembly 12, as shown in FIGS. 2A and
2B, are anti-reflection coated for the wavelength region between
780 nm and 800 nm, and are held in place within an aluminum housing
(not shown).
In operation, the lenses 31-37 collectively operate to collect and
focus about 80% of the light from the laser diode bar 11 onto FOCAL
POINT A. It should be noted at this time that the active medium 14
of solid state laser 13 is positioned at FOCAL POINT A to maximize
the absorption by the active medium 14 of the 785 nm light from the
laser diode bar 11. Additional information on the lenses 31-37 are
shown in the following TABLE.
TABLE
__________________________________________________________________________
Focal Lens Length Center Catalog Number Type (in mm) Dia. Thickness
Company Number Coating
__________________________________________________________________________
31 PCX 5.0 12.5 2.9 Spindler 063420 Lightning CYL & Hoyer
Optical Corp. MLAR @ 785 nm 32 PCX 10.0 18.0 4.0 Spindler 063421
Lightning CYL & Hoyer Optical Corp. MLAR @ 785 nm 33 PCX 25.4
25.4 7.0 Melles 01LCP006 Melles Griot/ CYL X22 Griot 076 HEBBAR 34
BCX 50.0 25.0 5.1 Melles 01LDX103 Melles Griot/ SPH Griot 076
HEBBAR 35 PCX 50.8 25.4 5.2 Melles 01LPX113 Melles Griot SPH Griot
076 HEBBAR 36 PCX 35.0 25.4 9.6 -- -- Lightning SPH Optical Corp.
MLAR @ 780- 800 nm 37 PCX 10.0 8.0 3.9 Melles 01LPX005 Melles
Griot/ SPH Griot 076 HEBBAR
__________________________________________________________________________
The address of Melles Griot is 1770 Kettering St., Irvine,
Calif.
The address of Spindler & Hoyer is 459 Fortune Blvd. Milford,
Mass.
The address of Lightning Optical Corp. is 131 Hibiscus St., Tarpon
Springs, Fla.
FIG. 3 is a magnified view of FOCAL POINT A. The beams or lobes
designated with vertical lines illustrate the two lobes of one of
the 600 laser diodes in the laser diode bar 11. It should be noted
that, by and large, the left lobes from all of the 600 laser diodes
land substantially in the same place at the FOCAL POINT A, while
the right lobes from all of the 600 laser diodes land substantially
in the same place at the FOCAL POINT A. Thus, FIG. 3 indicates that
the output of the laser diode bar 11 is to be focused into two
spots 41 and 43 that are approximately 440 .mu.m wide,
approximately 110 .mu.m thick, and separated by approximately 950
.mu.m. This two-spot pattern arises from the two-lobed far-field
patterns of each of the approximately 20 laser phased arrays in the
laser diode bar 11.
The performance of the lens assembly 12, as shown in FIGS. 2A and
2B, is evaluated by first scanning a 500 .mu.m aperture across the
FOCAL POINT A spot focused by the optics (FIGS. 2A and 2B) from a
laser diode, and then deconvolving the aperture function from this
profile. FIG. 4 shows this deconvolved profile as well as a
best-fit by a function consisting of the sum of two gaussian
functions. The observed performance of the lens assembly 12 of
FIGS. 2A and 2B agrees closely with the design expectations. The
lens assembly 12 of FIGS. 2A and 2B focuses more than 80% of the
output power from the laser diode bar 11 having a 1 cm aperture
into two spots approximately 150 .mu.m thick, approximately 680
.mu.m wide and separated by approximately 730 .mu.m. The
depth-of-focus of each spot is approximately 1 mm. The
discrepancies between expected and observed focusing properties are
attributed to the deviation of the actual laser diode output from
the idealized output assumed in the modeling. The ray-trace
simulations suggest and the experimental observations confirm that
the two spots are insufficiently resolved to attempt to overlap
them exactly using, for example, a dihedral prism.
The system of the invention images in the large divergence
direction, thus the average laser diode spatial distribution is
reproduced at the focused spot (FOCAL POINT A), and the spot size
is determined by the magnification ratio of the optics shown in
FIGS. 2A and 2B. As the emitted beam from laser diode bar 11 is
nearly diffraction-limited in the large-divergence direction, the
spot size in this direction is limited primarily by the divergence
direction (as shown in FIG. 2A), so the spatial distribution of the
light at the focused spot in that direction corresponds to the
far-field pattern of the laser diode bar, and is limited by the
magnification ratio of the optics. Thus, further adjustment of the
design of the optics (shown and described in relation to FIGS. 2A
and 2B) will not improve the shape of the spot. However, the spot
can be made smaller and more intense by increasing the
diffraction-limited aperture of the optics and decreasing the
magnification ratio of the optics in the system of the
invention.
FIG. 5 illustrates an exemplary plot of the output energy of the
Tm.sup.3+ :YAG laser of FIG. 1 as a function of the absorbed input
energy derived from the optical energy focused at FOCAL POINT A
(FIGS. 2A and 2B). The threshold is 4.4.+-.0.4 mJ and the slope
efficiency is 24.+-.1%. A maximum output energy of 1.3 mJ is
produced from 9.36 mJ of absorbed energy.
The invention provides the following advantages:
1. Off-the-shelf optical parts in the lens assembly 32 (shown in
FIGS. 2A and 2B) are preferrably utilized on the basis of cost and
availability. The lens assembly of FIGS. 2A and 2B, and the
associated TABLE, use a progression of plano-convex cylindrical
lenses in order of increasing focal lengths, followed by
plano-convex spherical lenses in order of decreasing focal lengths
in order to realize an optical system having a high numerical
aperture and a short effective focal length.
2. It minimizes aberrations in the large-divergence direction (FIG.
2B). This approach is realistic because the small emitting aperture
in the large-divergence direction (approximately 1 .mu.m) renders a
beam comparatively close to the diffraction limit.
3. It uses the shortest focal length optics practical in order to
minimize spread in the low divergence direction (FIG. 2A).
4. It takes advantage of the two-lobe laser diode far-field pattern
to obtain two spots (41 and 43 in FIG. 3) of higher intensity than
possible without the two-lobe pattern. It specifically treats each
laser diode in the laser diode bar 11 as if it is emitting two very
narrow (approximately 2 degree) divergence beams (separated by
approximately 6 degrees) rather than a single output beam having a
larger divergence of approximately 8 degrees.
Therefore, what has been described in a preferred embodiment of the
invention is a system for focusing the output bright light from an
elongated area of a long aperture laser diode bar into a small,
high intensity spot to end pump a solid state laser.
It should therefore readily be understood that many modifications
and variations of the present invention are possible within the
purview of the claimed invention. For example, any other type of
active medium 14 could be utilized in the invention, such as a
Ho.sup.3+ :YAG. While the invention is particularly useful in
end-pumping 3-level lasers and self-terminating lasers, it could
also be used to end-pump 4-level lasers. In addition, the optical
system shown in FIGS. 2A and 2B could be replaced with a different
set of optics such as, for example, by one or two compound lenses
or even more than the 7 lenses shown in FIGS. 2A and 2B. However,
such replacement lenses could be very expensive to fabricate. It is
therefore to be understood that, within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described.
* * * * *